1. Field of the Invention
The present invention relates to a method for cleaning the U-tube of the measurement cell of a densimeter.
2. Description of the Related Art
Of the physical measurements which have to be effected in the context of industrial processes, that of density figures among the most frequently necessary.
To that end, manufacturers market a range of densimeters based on various principles, all of which have advantages and disadvantages.
A densimeter which can be used in a satisfactory manner to measure the density of a sample is equipped with a measurement cell comprising the following elements:                an isothermic enclosure which defines at its inner portion a measurement chamber closed by a stopper surrounded by a resilient seal;        a U-tube which is to be filled with a sample being analyzed and which extends inside the measurement chamber, the U-tube being secured to the stopper at its base, which is formed by its free ends which project outwardly from the measurement chamber in order to permit the injection of the sample being analyzed via an injection opening and its evacuation via an evacuation opening; and        means for causing the U-tube to vibrate, as well as means for measuring the vibratory response of that tube.        
By way of example, the U-tube can be kept earthed and the means for causing the tube to vibrate may be formed by:                a ferromagnetic member with which the U-tube is equipped at its middle portion;        an insulated conductive reading plate maintained at a difference in potential relative to the U-tube and secured to the stopper in a position facing that tube in order to define a capacitor therewith; and        an electromagnetic excitation winding mounted at right-angles to the ferromagnetic member.        
The principle of measuring the density of a sample by means of such a densimeter consists in causing the U-tube to vibrate at resonance frequency and in determining that frequency from the variations in the voltage at the terminals of the capacitor.
The resonance frequency enables the density of the sample being analyzed to be calculated to a first approximation on the basis of a standard equation known per se and from a preliminary calibration of the densimeter.
After each analysis, it is necessary to clean and rinse the U-tube of the measurement cell very carefully in order not to impair the subsequent analysis owing to the presence of sample residues deposited on the walls of the tube.
Such cleaning is a particularly long and inconvenient operation in as much as it requires the implementation of several successive steps.
The first of those steps consists in injecting via the injection opening of the U-tube, under pressure or by suction, a first solvent or rinsing solvent which must necessarily be compatible with the sample previously analyzed and evacuated via the evacuation opening, that is to say, it must be capable of dissolving that sample.
The rinsing solvent can be injected in the form of a homogeneous flux or in the form of a foam.
After the evacuation of the first solvent via the evacuation opening, it is necessary to inject via the injection opening a second, volatile, solvent or drying solvent which must then likewise be evacuated via the evacuation opening before the tube is dried by ventilation.
Such a method has a large number of disadvantages, the first of which is associated with the necessity to use significant amounts of two different solvents.
The amount of rinsing solvent which must be used is particularly large in the case of viscous and/or adhesive samples.
In addition, when different samples which are immiscible with one another are analyzed, it is necessary to use different types of rinsing solvent.
Added to that is the necessity periodically to carry out scaling operations in order to eliminate the solid deposits which have adhered strongly to the walls of the U-tube.
Those operations require the use of reagents based on very toxic strong acids, such as, by way of example, hot chromic acid solutions which are particularly aggressive and also carcinogenic.